The present invention relates to apparatus for projecting a screen pattern onto a light-sensitive surface and, more particularly, to compact apparatus for projecting a line screen pattern onto a photoconductor of a duplicator or the like from a point light source.
The use of screen patterns and screening processes to improve the rendition of images is well known in the fields of photography and electrography. See, for example, U.S. Pat. No. 3,493,381, which issued in the name of R. E. Maurer on Feb. 3, 1970, and U.S. Pat. No. 2,598,732, which issued in the name of L. E. Walkup on June 3, 1952. These patents also disclose a variety of screen patterns including lines, circular or rectangular dots, checkerboard patterns, etc.
The optical system disclosed in U.S. Pat. No. 3,936,173 which issued on Feb. 3, 1976 in the names of W. L. Kidd et al, is used for projecting a screened light pattern onto a photoconductor that also receives a light image of an original document. The screening apparatus includes an electroluminescent panel that is energized from a suitable power supply and the resulting light rays are projected from that panel through a collimator or lens strip. The resulting collimated light rays are then transmitted through a screen having a pattern of lines or dots thereon to form a screened light image which is projected onto a photoconductor.
In some of the prior art devices the light source can be located at a convenient distance as required by the optical system. In other instances, such as in the U.S. Pat. No. 3,936,173, special light collimators are provided for handling light from elongate luminescent sources in order to provide the desired collimated light beam to a dot or line screen. However, in some electrophotographic apparatus there is very little space or area available within the apparatus to place a screen projector. In addition, it frequently is desireable to provide a small, concentrated point light source as the projection lamp instead of elongate lamps. In accordance with the present invention the desireable collimated light rays are provided to a screen pattern over a narrow, rectangular aperature within a restricted space envelope, and such is accomplished while using a small, concentrated point light source.